The present invention relates to a flexible thermal transfer apparatus for cooling electronic components. In particular, the present invention relates to a two-phase, flexible thermal transfer bag containing a low boiling point thermal transfer liquid which vaporizes and then condenses in response to heat flux from a heat source.
As electronic systems become more compact, various auxiliary heat transfer techniques have been proposed to satisfy a need for increased system cooling. Heat transfer techniques have conventionally employed fans, heat sinks or both. Fans dissipate thermal energy from components by forced air convection. Passive heat sinks dissipate thermal energy by natural air convection.
With conventional heat transfer techniques, it is sometimes difficult to provide sufficient cooling for components within compact or densely arranged electronic systems. Forced air convection methods have practical limits because the amount of air required to provide sufficient cooling generally creates an unacceptable level of noise. Also, forced air convection methods have difficulty maintaining a large number of electronic components within critical, narrow operating temperature ranges. Passive heat sinks require a specified volume which may be difficult to provide within compact or densely arranged electronic systems.
In a circulating liquid cooling system, the circulating liquid is in thermal communication with the heat source and transfers heat to a remote heat sink, such as a chilled water supply, a refrigeration system or a liquid-to-air heat exchanger. Although these systems may be effective where passive or forced air convection techniques are insufficient, these systems are often complex, expensive and can significantly add to the overall system size.
In large scale electronic systems like super-computers or avionics systems, liquid immersion heat transfer techniques have been employed using a fluorocarbon liquid as the cooling medium. For example, Danielson et al., Cooling a Superfast Computer, Elec. Packaging and Prod. pp. 44-45 (July 1986) discloses a method of cooling a supercomputer, the Cray-2.TM., by immersing the computer, power supplies, memory board, logic circuits and main processors in a sealed bank of circulating inert, high-dielectric, perfluorocarbon, Fluorinert.RTM. electronic liquid FC-77.
Another liquid immersion heat transfer technique is described in U.S. Pat. No. 3,74 1,292 (Aakalu et al.) In this technique, a rigid container is mounted to a substrate having a plurality of microelectronic components to be cooled. The container encloses the heat generating components and includes a sufficient volume of low boiling point, dielectric liquid to partially fill the container and immerse the components. The container further includes a vapor space located above the liquid. A plurality of internal fins extend inward into the vapor space, which serve as a condenser for the dielectric vapors. A plurality of external fins extend outward from the container, which serve as an air-cooled heat sink for the internal fin condenser. Aakalu et al. suggest the low boiling point dielectric liquid can include one of the fluorocarbon liquids FC-78 or FC-88. Aakalu et al. suggest that since the container is sealed, a binary mixture of fluids having different characteristics can be selected to give the minimum amount of pressure buildup in the container.
As described in the Aakalu '292 patent, the low boiling point dielectric fluorocarbon liquids give rise to various types of boiling at relatively low temperature. The mode of boiling and the resulting heat transfer are dependent on the heat flux at the interface between the component to be cooled and the coolant liquid. For a small heat flux that results in a temperature below the boiling point of the liquid, natural convection takes place. As the heat flux increases, the temperature increases beyond the boiling point of the liquid which causes nucleate boiling. Nucleate boiling causes the liquid to vaporize immediately adjacent the heat source. As vapor bubbles form and grow on the heated surface, they cause intense microconvection currents and remove heat by the latent heat of vaporization of the liquid. Thus, nucleate boiling increases convection within the liquid and improves heat transfer between the heat source and the liquid. The system disclosed by Aakalu et al. operates similar to a rigid heat pipe.
A disadvantage of the rigid container disclosed in the Aakalu et al. '292 patent and of rigid heat pipes in general is that the boiling point of the heat transfer liquid in the container varies with the internal pressure of the container. As the power dissipated from the heat generating component increases, the heat flux from the component increases which increases vaporization. Since the container has fixed, rigid walls, the internal pressure of the container increases which causes the boiling point of the heat transfer liquid to increase. As a result, the operating temperature of the component increases with the internal pressure of the container. Another disadvantage of fixed, rigid containers and of heat pipes in general is that they consume a substantial amount of space and are relatively heavy.
A recent development which efficiently transfers heat in compact electronic systems is a Liquid Heat Sink.TM. thermal transfer bag available from Minnesota Mining and Manufacturing Company, assignee of the present invention. The thermal transfer bag is made of a sheet of flexible, durable, air-impermeable material. The bag is filled with a thermally conductive, chemically inert, essentially gas-free fluorochemical liquid. The bag is placed within an internal cavity of the electronic system, between a surface of a heat generating component and a heat dissipating surface. Conduction through the liquid as well as some movement of liquid within the bag due to convection currents transfer heat from the heat generating component to the heat dissipating surface. The liquid has a boiling point which is high enough that the liquid will not boil at the highest operating temperature of the heat generating component.
The thermal transfer bag readily conforms to the internal cavity and comes into intimate contact with the heat generating component and the heat dissipating surface. The thermal transfer bag does not require additional battery power to cool the heat generating components. In some applications, the inherent shock-absorbing nature of the filled bag protects the components from physical shock. The bag can easily be removed and replaced in the field during repair, and may optionally be provided with an adhesive to hold the bag in place. U.S. Pat. No. 4,997,032 (Danielson et al.) describes the thermal transfer bag in greater detail.
U.S. Pat. No. 5,000,256 (Tousignant) discloses a thermal transfer bag having a metallic thermal via. The metallic thermal via extends through an aperture in the bag for contact with an external heat generating component. A portion of the via extends into the bag and functions as a heat radiating fin to enhance heat transfer from the heat generating component to the liquid within the bag.
U.S. Pat. No. 5,046,552 (Tousignant) discloses a heat transfer apparatus having a frame with a channel for the flow of heat transfer liquid. The apparatus further includes a flexible sheet connected to the frame and a thermal via coupled to the sheet. As fluid flows through the channel, the sheet and via move outwardly and contact the component to be cooled. A cover structure extends across the frame adjacent the via and limits outward movement of the sheet. The cover structure also prevents damage to the sheet when the apparatus is handled.
Australian Patent No. 75381/91 discloses a heat pipe in the form of a panel comprising an enclosed chamber having opposing, generally planar, parallel wall portions. The chamber is filled with a liquid and a vapor of the liquid, with the liquid and the vapor being in equilibrium. When one of the wall portions is heated, the vapor transfers heat from the wall portion being heated to the other wall portion where the vapor condenses. Both walls portions may be rigid or flexible. The chamber is also filled with particulate matter, such as glass beads, to prevent deformation when the vapor pressure is less than atmospheric or less than the surrounding atmosphere.
Other thermal transfer techniques have also been employed. U.S. Pat. No. 4,092,697 (Spaight) discloses cooling an integrated circuit package through a liquid contained in a film mounted on the underside of a cover enclosing the integrated circuit. The cover, film and liquid form a formable pillow such that when the cover is sealed to the package, the pillow contacts the top of the integrated circuits mounted within the package.
U.S. Pat. No. 4,155,402 (Just) discloses a means of packaging a printed circuit board. Circuit components are cooled by a liquid-cooled cold plate having a compliant mat interface. The interface is made of a film bonded to the underside of a liquid cold plate. Contained between the cold plate and the film is a paste such as thermal grease which can contain metal particles.
U.S. Pat. No. 4,563,375 (Ulrich) discloses a flat bag made of foils, such as aluminum, filled to only part of its maximum available volume with a thermally conductive paste without the presence of gas. The bag is disposed between substantially planar surfaces or vertical slats as a means of heat transfer.
There is a continuing desire to increase the rate of heat transfer away from the heat generating components of an electrical device and to decrease the thermal resistance through the components and the heat sink. Increasing the rate of heat transfer becomes even more important in instances where the components produce relatively large amounts of heat or where there is a need to reduce the overall size of the device. By increasing the rate of heat transfer, heat generating components may be spaced closer together with other components while still enabling each component to function properly within a stable, desired temperature range of operation.